In the present study, we aimed to investigate changes in the mineral composition and solubility of rat bone under various regimens of fluoride administration in drinking water. Sprague-Dawley rats (male, 4-week-old at the beginning of fluoride administration) were used. We adopted two animal-housing protocols: (1) the age-matched animals were given 0ppm (control) or 50ppm fluoride as NaF in deionized water for various periods ranging from 2 to 16 weeks, and (2) the animals were housed for 10 weeks under fluoride regimens of 0 (control), 10, 30, 50, 70 or 90ppm. At the end of fluoride administration, the animals were sacrificed and then the diaphysial cortical bones of the femora and tibia were harvested. All bone samples were pulverized and deproteinated by low-temperature ashing prior to use in solubility measure-ments. The solubility of bone crystals was determined through a series of selid/solution equilibration at 25°C under 1.8% CO₂/N₂ gas environment. The results obtained showed that bone mineral composition was highly sensitive to the ingested fluoride, increasing the fluoridation degree of bone crystals up to a plateau around 1 wt%, i. e., one third of the theoretical content for fluorapatite. From the solubility data of bone samples collected according to both animal-housing protocols, it was proved that bone crystal solubility improved most substantially during the initial 6 weeks of fluoride administration and in the concentration range lower than 30-ppm fluoride in the drinking water. Further improvement of the solubility was only modest in magnitude (as indicated by changes in solubility product) even after fluoride ingestion over longer administration periods or at concentrations of 50-90ppm fluoride.